A rotary head type magnetic recording/reproducing device, such as video cassette recorder (VCR) or a digital audio tape recorder (DAT), has mounted therein a rotary center-shaft type magnetic head drum whose center shaft rotates together with the rotary drum or a stationary center-shaft type magnetic head drum whose center shaft is secured to a stationary drum.
The rotary center-shaft type magnetic head drum assembly is composed of a rotary drum provided with, at least, a magnetic head, a rotary side transformer coil of a rotary transformer, a rotor and a center shaft of a motor and a stationary drum provided with, at least, a fixed side transformer coil of the rotary transformer and a stator of the motor.
The stationary center-shaft type magnetic head drum assembly is composed of a rotary drum provided with, at least, a magnetic head, a rotary side transformer coil of a rotary transformer, a rotor of a motor and a center shaft and a stationary drum provided with, at least, a fixed side transformer coil of the rotary transformer, a stator of a motor and a center shaft.
The rotary drums and the stationary drum are usually manufactured by a casting or forging process for preforming configurations of products and a machining process for cutting, drilling, tapping and finishing the cast or forged products. Because of the number of manufacturing steps, these drums are expensive parts among components of a complete magnetic recording/reproducing device. It is, therefore, strongly desired to realize a considerable reduction in their manufacturing cost.
To realize this, attempts have been made to manufacture the rotary and stationary drums by precision injection molding or precision compression molding from high-polymer resin material. The precision injection molding and the precision compression molding are respected to bring a considerable reduction in costs of the drums because both methods can save steps and time for manufacturing the products. However, these drums must be finished to attain an accuracy on the order of one micron in its size and shape. Such a high accuracy can not be obtained by the precision injection or compression molding only since molded products may have sink, warp and creep deformation.
In view of the above-mentioned problems, Japanese Laid-Open Patent Publication No. 4-95246 proposes a drum that has a body made of material such as an aluminum die-cast with resin layers formed on a tape-guiding surface and an internal surface of a bearing bore. It is said that the drum can be formed by precise compression molding without additional machining process and therefore it can be produced at a considerably reduced cost.
Japanese Laid-Open Patent Publication No. 64-59611 discloses a drum which has a core housing made of metal or ceramics with a tape-transport portion formed thereon from resin material. This enables the drum to be produced at a considerably reduced cost because the precise machining process is eliminated. Furthermore, its resin-made portion is thin enough to keep a high accuracy of size and shape with reduced sink, warp and creep as compared with a whole resin-made product. The metal- or ceramic-made core-housing of the magnetic head drum is rigid enough to be reliably fixed on its base. Namely, Japanese Laid-Open Patent Publication Nos. 4-95256 and 64-59611 realize savings in manufacturing cost of the drum products by forming only particularly accurate portions (e.g., tape-transport or tape-guiding portion and bearing bore surface) from resin material, thus eliminating the need of machining metal or ceramic material. However, the magnetic head drum assembly also requires high accuracy of other portions (e.g., fixing parts) which must be additionally machined in practice. As a matter of fact, the drum assembly is still expensive.
Japanese Laid-Open Patent Publication No. 3-89513 discloses a rotary drum which is formed from resin material with inserted members enclosed thereby. This may simplify the manufacturing process because the portions required to be accurate in size and shape are formed from resin material.
However, the rotary drum described in Japanese Laid-Open Patent Publication No. 3-89513 has a number of drawbacks from the viewpoint that it is a component to be assembled into a magnetic head drum unit whose performance has to meet the technical requirements of a completely assembled product. Namely, the magnetic head drum assembly involves the following problems:
A first problem lies in that the magnetic head drum, secured at its resin-made portion to a base plate (substrate), may be subjected to not-negligible long-term deformation of resin material due to creep or internal stress therein, resulting in deterioration of initial size and shape accuracies. The same problem may occur in a magnetic head mounting portion of the rotary drum.
A second problem lies in that a rotary drum and a stationary drum, which are formed from resin material (crystalline polymer in particular), has a poor ability of bonding thereto a rotary-side transformer coil and fixed-side transformer coil, respectively, of a rotary transformer. Necessary bonding strength can not be obtained.
A third problem is that the magnetic head drum formed from resin material can not be sufficiently shielded against EMI (electro magnetic interference) because resin material filled with conductive filler can attain conductivity of 10.sup.3 .OMEGA. (not exceeding the limit of semi-conductor). As a result of this, it may reproduce a signal containing electromagnetic noise that deteriorates the reproduced image quality.
A fourth problem lies in that the magnetic head drum requires reliable fixture and support of its bearing with a center shaft (for the rotary center-axis type) or its center shaft with bearing (for the fixed center-axis type) but it can not maintain for a long term the initial size and shape accuracies and impact strength enough to withstand a drop test with acceleration of several hundreds of G because of the above-described deterioration of the resin material under the influence of creep and internal stress.